The 1N5711 diodes used in the AA908 Reflectometer, and as positive amplitude
clippers on U3 will have slightly different forward Vf curves one to
another.. The lower the current thru them the closer to the steep knee
and greatest nonliniarity experienced.When 50 ohms is used, the internal impedence of
the reflectometer is close to 50 ohms so the currents in the legs of the
reflectometer is nearly evenly balanced.
However as the resistive or reactive loads are increased the currents thru
the 1N5711 diodes will also decrease for some and then the lesser current,
and the greater nonlinearity experienced will create a mismatch error with
sensativity to small self heating changes and changes in Vf of the diodes.

It is some of this that is effecting your readings as a function of
frequency and time from turn on

This has been a never ending problem with full wave diode multiplexers
and demuxers for years.

Good ones have all diodes of the same build lot. then selected
to match the Vf curves as well potting them all tightly together so that the
temperatrue of all are nearly the same. Still some nonlinearity match
effects sneak in at low or very low signal and current levels.

In some High accuracy needs -applications, a exteranal adjustabe bios is
established for each of the 4 diodes of a Detector or Demuxer and that is
used to set the operation currents thru the diodes at points along the
flatest part of the Vf curves where it is easier to match the set of diodes.

FROM: Joe Everhart, N2CX ...

Indeed there may be some non-matching issues with
the diodes we use in our detectors. That's one of the things we are
investigating.

I'm not sure what you mean by "positive amplitude clippers".
You may be describing the prebiased diodes in the feedback loops on the
amplifiers fed by the diode detectors. This compensation method has been
used for quite a while in RF power meters and have been described in some
detail in the ham literature:

We chose the diode current to give reasonable performance at low signal levels
where the diodes are indeed quite non-linear.

Our original prototype testing and circuit simulation seemed to indicate that
matching of the diodes was not needed for reasonably good performance.
We are undergoing comparative testing to identify the mechanism for the
observed errors in measurements at other than 50-ohm situations. None of
the testing we have done to date indicates any frequency effects on the
detectors in the HF region. The major cause for error to date has been
input-level related errors in the detectors with our simple circuitry.

The interesting thing is that the multiple prototypes and breadboards that we
have set up with the current circuit configuration and software do not seem to
exhibit the problems to the degree noted by some in the field. We are
attempting to duplicate the same effects to study them.

The bottom line is "stay tuned!"

FROM: George Neron, N2APB
...

Indeed this whole area of low-level detection,
and the techniques we used to implement it in the Micro908 reflectometer and
buffers, was strongly considered during the 2-year design cycle for the
product. In fact, we provide similarly-detailed analysis and data in
the Technical Reference Manual located online at the project web page http://www.amqrp.org/kits/micro908.
Sure hope all kit owners have read through that document, as it provides
some great insight to the design solutions.

That all said, George Silver is right on the
mark regarding the challenges confronting designers looking to reliably
detect and correlate multiple-channel, low-level RF signals. The
technique that Joe implemented in his design for the reflectometer, as
cited in the Grebenkemper/Lewallen references below, utilizes a diode in the
first buffer amplifier of each channel to compensate for the nonlinearity
presented by the diode upstream in the reflectometer. In other
words, the nonlinear current-voltage response of the reflectometer
diode is counteracted by the slight change in the near-unity gain of the
first amplification stage caused by the use of the same diode in the
feedback loop. If the reflectometer diode is a bit higher than it
should be (because of the nonlinear response near its low-end
"knee"), the gain of the first buffer will be slightly lower by
the same amount because that feedback diode is also operating at that same
low-end "knee".

This type of compensation would produce perfectly
linear response through the whole signal chain *only* when the diode
characteristics are absolutely identical. Of course we all know this
"identical diode characteristics" condition doesn't happen in the
real world, so we get as close to it as possible by using good diodes with
low threshold "knees" (like the 1N5711 Shottky diodes), and providing all
diodes in each kit from the same batch as received from the vendor.
It's a reasonable assumption that components along the SMT reel tape are
provided from the same manufacturing batch from the original manufacturer,
so the characteristic of the diodes are likely as close as possible to each
other. Further, we purchased all diodes for 1000 kits at the same
time, thus ensuring a good likelihood for very close characteristics for
this all-important component.

BTW, we did a detailed experiment upfront in the
first kitting round in order to determine the sensitivity of instrument
measurement accuracy vs. diode response. We measured a large batch of diodes
and sorted them according to their low-level voltage characteristic, then we
used those diodes with near-identical response in a Micro908. The
measurements obtained from this test controlled-kitted unit were nearly
identical to a standard-kitted Micro908, so we felt that the
"compensated design" approach was indeed valid.

Now, after going through all this, the situation
is that some kit owners are experiencing some measurement variability at
higher-resistive loads (like when using the 274-ohm resistor).
Several factors could contribute to this condition ...

1) diode nonlinearities at low signal levels -- discussed above.

2) math inaccuracies &amp round-off -- studied before but revisiting.

3) calibration software (mis)assumptions -- studying again.

4) calibration approach -- studying again.

So you can see that our current focus is on the
math and calibration implementations. Joe and I are currently
revisiting this whole area of calibration to see if we can indeed provide
some improvements to the instrument's upper-end accuracy so that *all* units
experience the same measurement quality that most of them have already.
Most of you will recall several different calibration approaches that I had
previously implemented - the most notable being the
"multi-multi" technique that required many adjustments for each of
several different load resistors. There may be an adaptation of this
calibration technique that will work better/easier, now that we've solved
the "spectral purity" issues by adding the DDS Amp.

So as Joe said "Stay Tuned" ... the
pun was intended of course, by pun-master N2CX.

And thanks again to George Silver (et al)
for his insight on the topic ... this is an "open design" and we
certainly welcome all input, suggestions and experience on the technical
issues of the Micro908. This is what will enable the product to
continually evolve and continue to be better than anything else on the market in this
price range.

Material and
concepts presented on the AmQRP website is Copyright 2005 by the American QRP
Club, Inc.
These pages are designed and maintained by George Heron, N2APB
(n2apb_at_amqrp.org)
Page last updated: March 19, 2005